This application claims priority under 35 U.S.C. xc2xa7xc2xa7 119 and/or 365 to Japanese Application No. 9-155981 filed in Japan on May 28, 1997; the entire content of which is hereby incorporated by reference.
1. Field of Industrial Utilization
The present invention relates to a sputtering method used in the fabrication of various types of semiconductor devices. More particularly, it relates to an ionizing sputtering method utilized in the formation of films by the ionization of sputter particles.
2. Description of Related Art
With semiconductor devices, such as various types of memory and logic devices, a sputtering process is used in the formation of various wiring films, and in the production of barrier films that prevent the interdiffusion of different layers, and sputter devices are commonly used for this purpose. There has recently been a great need for such sputtering devices to allow the inner surfaces of holes formed in a substrate to be coated with a good degree of coverage.
A CMOS-FET (field effect transistor), which is commonly used in a DRAM, employs a structure that prevents cross-contamination between the contact wiring layer and the diffusion layer by the provision of a barrier film to the inner surface of the contact holes provided on the diffusion layer. With a multilayer wiring structure for the wiring of memory cells, through holes are provided to an interlayer insulation film, and interlayer wiring is embedded inside these through holes in order to link the lower layer wiring with the upper layer wiring. Here again, a structure is adopted in which a barrier film is produced inside the through holes to prevent cross-contamination.
Because of the increasing degree of integration, the aspect ratio of these holes, i.e., the ratio of the hole depth to the size of the hole opening, has been steadily rising over the years. For example, the aspect ratio is about 4 with a 64 megabit DRAM, and is about 5 to 6 with a 256 megabit DRAM.
In the case of a barrier film, a thin film must be built up on the bottom of the hole in an amount of 10 to 15% of the amount of build-up on the peripheral surfaces of the hole. For holes with a high aspect ratio, it is difficult to form a film with a high bottom coverage, i.e., the ratio of the deposition rate on the hole bottom to the deposition rate on the peripheral surfaces of the hole. A decrease in the bottom coverage can lead to a thinner barrier film at the bottom of the hole and to critical flaws in the device characteristics, such as junction leakage.
Collimation sputtering and low-pressure, long-distance sputtering processes have been developed up to now as sputtering processes that increase the bottom coverage. Collimation sputtering involves using a plate (collimator) in which numerous holes have been made in the direction perpendicular to the substrate, and providing this plate between the target and the substrate. Collimation sputtering is a process in which only those sputter particles, usually sputter atoms, that fly more or less perpendicular to the substrate are selectively allowed to reach the substrate. Low-pressure, long-distance sputtering involves lengthening the distance between the target and substrate (usually about 3 to 5 times farther) so that relatively more of the sputter particles that fly more or less perpendicular to the substrate will land on the substrate, and lowering the pressure more than usual (about to 0.8 mTorr or less) so that the free mean path is longer, which results in less turbulence of these sputter particles.
A problem with collimation sputtering is that sputter particles accumulate on the collimator portion, and the resulting loss of material decreases the deposition rate. A problem with low-pressure, long-distance sputtering is that since the pressure is lowered and the distance between the target and the substrate is lengthened, there is a fundamental decrease in the deposition rate. Because of these problems, collimation sputtering is only used for mass-produced products of the 16-megabit class in which the aspect ratio is up to about 3, and low-pressure, long-distance sputtering is limited to devices up to an aspect ratio up to about 4.
In light of the above, an ionizing sputtering process was investigated as a technique that would allow a film to be deposited with good bottom coverage for holes whose aspect ratio is over 4. Ionizing sputtering is a process that ionizes the sputter particles released from a target, and raises bottom coverage by the action of these ions.
However, a number of practical problems are encountered with ionizing sputtering. One of these is the structure of the energy supply used for ionization.
An effective way to perform ionizing sputtering is to form a plasma along the flight path of the sputter particles from the target to the substrate. To form this plasma, an electrode in the form of a coil or a sheet is provided separately from the target, and this electrode is connected to a power source that applies electric power.
A drawback to a structure such as this, however, is the complexity of the structure inside the sputter chamber. On top of this, a power source is provided separately from the sputtering power source, so the structure around the periphery of the sputter chamber is also complicated, and the high cost is another problem.
The invention of this application was conceived in an effort to solve these problems, and an object thereof is to provide a method with which a film can be deposited by ionizing sputtering with good bottom coverage for holes with a high aspect ratio.
Another object of the present invention is to provide a method with which the structure inside and outside the sputter chamber can be simplified and the cost lowered.
An ionizing sputtering method according to the present invention includes the steps of maintaining the inside of a sputter chamber at a pressure between 10 and 100 mTorr; applying electric power to a target provided inside the sputter chamber so as to create a sputter discharge and sputter said target to release sputter particles from the target, wherein the sputter particles are ionized only by the electric power applied to the target in a plasma formed by the sputter discharge; and making the sputter particles released from the target arrive at a substrate so as to build up a thin film on the surface of the substrate.
Another aspect of the the present invention includes an ionizing sputtering method comprising the steps of maintaining the inside of a sputter chamber at a pressure between 20 and 100 mTorr; applying a high frequency electric power to a target provided inside the sputter chamber so as to create a sputter discharge and sputter said target to release sputter particles from the target, wherein the sputter particles are ionized only by the high frequency electric power applied to the target in a plasma formed by the sputter discharge; and making the sputter particles released from the target arrive at a substrate so as to build up a thin film on the surface of the substrate; wherein a power area density of the high frequency electric power divided by a surface area of the target being sputtered is at least 5 W/cm2.
Yet another ionizing sputtering method according to the present invention comprises the steps of applying a high frequency electric power to a target provided inside the sputter chamber so as to create a sputter discharge and sputter said target to release sputter particles from the target, wherein the sputter particles are ionized only by the high frequency electric power applied to the target in a plasma formed by the sputter discharge; and making the sputter particles released from the target arrive at a substrate so as to build up a thin film on the surface of the substrate; wherein a power area density of the high frequency electric power divided by a surface area of the target being sputtered is at least 5 W/cm2.